How to add add points on boxplot

I'm confident we can help you, but you will need to give us more information. Are you making the box plot from the build in specialty chart type (which may not have any flexibility to add additional series) or are you using the "old-fashioned" method of using a stacked column/bar chart?

One thing I would do is to transpose your additional data so that it is oriented the same as the main data (months in ascending order in a row with the corresponding follows in the row beneath). It may not always be necessary, but I find it helpful to be consistent with the way my data are arranged -- in this case, have all data arranged in rows.

It's not clear to me what you mean by "add these points to the diagram".

If you mean that you want to add them to the data so that they are included in the chart's data analysis, copy and transpose the new data into row 21 and extend the chart's source data so it includes row 21. Because that seems so trivial, I am guessing you want something else, but I include this for completeness.

If you mean that you want to add these as a separate "data series" (probably as a line chart type so that you get a marker at these points), then I don't think that works in the new specialty chart types. This effectively creates a "box plot + line" combination chart, but as I understand it, Excel will not allow you to make any kind of combination chart with a box plot specialty chart. If that proves to be the case, then I would suggest that you make the chart "the old-fashioned way" -- as a stacked column (+line for the extra data series) chart. This tutorial should walk you through the process: https://peltiertech.com/excel-box-an...ams-box-plots/